As a water level measuring apparatus that measures the water level of a liquid stored in a liquid tank, there has been proposed a technique for indirectly measuring the water level based on a change in an interelectrode capacitance of a pair of electrodes arranged with a predetermined gap therebetween, as disclosed in Japanese Laid-Open (Kokai) Patent Application Publication No. H11(1999)-311561. Moreover, as a water level measuring apparatus that measures the water level by using another principle, there has been proposed a technique for directly measuring the water level based on a position of a float floating on a water surface, as disclosed in Japanese Laid-Open (Kokai) Patent Application Publication No. 2006-170927.
However, in the conventionally proposed water level measuring apparatus, when the surface is stable, the water level can be accurately measured, but when the surface is sloshing, the water level fluctuates up and down, and it is very difficult to determine which fluctuating value should be adopted as the water level. Particularly, when the water level of a liquid such as fuel carried in a moving vehicle is to be measured, multi-directional acceleration acts on the liquid so that the surface sloshes irregularly, and hence this undesirable situation may appear noticeably.